Heat-developable photosensitive material is well known in the art. The heat-developable photosensitive material and its process are described in the literature and patents, for example, "Shasinkougaku No Kiso" --Higinen Shasin-- ("Fundamentals of Photographic Engineering" --Non-Silver Salt Photography--), 1982, Corona Publishing K.K., pages 242-255 and U.S. Pat. No. 4,500,626 which is incorporated herein by reference. In addition, a method of forming dye images through coupling reaction with an oxidant of a developing agent is described in U.S. Pat. Nos. 3,761,270 and 4,021,240.
It was also proposed to imagewise release or form a diffusible dye through heat development and transfer the dye to a dye fixing element. With this technique, either negative or positive dye images can be obtained by selecting a suitable type of dye-providing compound or a suitable type of silver halide. For detail, reference is made to U.S. Pat. No. 4,500,626, 4,483,914, 4,503,137, 4,559,290; Japanese Patent Application Kokai (JP-A) Nos. 149046/1983, 218443/1984, 133449/1985, and 238056/1986; EP 210660 A2 and 220746 A2; Japan Invention Society's Kokai Giho (Technical Report) No. 87-6199 and the like.
A variety of proposals have been made in the art for producing positive color images through heat development. For example, U.S. Pat. No. 4,559,290 proposes a method for forming an image by converting a dye providing (DRR) compound into an oxidized form having no dye releasing ability, preparing a heat-developable material in which the oxidized DRR compound is co-present with a reducing agent or a precursor thereof, carrying out heat development to oxidize the reducing agent in an amount corresponding to the exposure of silver halide, and allowing the remainder of the reducing agent unoxidized to reduce the oxidized DRR compound into the DRR compound to release a diffusible dye. EP-A 220,746 and Technical Report No. 87-6199 (Vol. 12, No. 22) describe a compound capable of releasing a diffusible dye through a similar mechanism, more particularly a heat-developable color photosensitive material using a compound capable of releasing a diffusible dye through reductive cleavage of an N-X linkage where X is an oxygen, nitrogen or sulfur atom.
It is known that the heat-development process often entails fog, known as thermal fog, which is not expectable from conventional wet development or processing in a liquid developer. The thermal fog is a serious problem against image formation. The thermal fog increases the minimum density in heat-developable photosensitive material of the negative type forming color images in proportion to latent images, resulting in white areas becoming tarnished. It decreases the maximum density in heat-developable photosensitive material of the positive type forming color images in counter-proportion to latent images. The thermal fog is one of main factors prohibiting sensitivity increase since it tends to occur particularly when an emulsion containing silver halide grains of a larger size is used in order to provide high sensitivity.
For preventing the thermal fog, it is a common practice in the prior art to use organic compounds known as antifoggants. Conventional antifoggants, however, suffer from several problems since some are less effective for suppressing fog and some suppress fog, but retard development at the same time.
Particularly when a photosensitive material having a spectral sensitivity peak in the infrared region is desired, it is prohibited to use a sufficient amount of an effective antifoggant since infrared sensitizing dyes do not generally provide so strong adsorption to silver halide. Differently stated, if an effective antifoggant is used in a sufficient amount, it would restrain adsorption of the sensitizing dye, resulting in reduced sensitivity. In addition, there is a potential serious problem that aging of an emulsion coating composition in solution form would cause desorption of the dye, resulting in reduced sensitivity. In this way, an attempt to suppress fog would aggravate the stability of an emulsion coating composition during solution form aging whereas an attempt to improve the stability of an emulsion coating composition during solution form aging by reducing the amount of antifoggant used would eventually invite increased fog. It was thus difficult in heat development to achieve fog suppression and to stabilize an emulsion coating composition during solution form aging at the same time.
For this reason and other, there exists no definite design guideline for heat-developable color photosensitive material to achieve high sensitivity and low fog while satisfying manufacturing feasibility. It is thus desired to develop a heat-developable color photosensitive material comprising an emulsion which is characterized by high sensitivity and low fog as well as the stability of its coating solution with the lapse of time.